"Phil Allison" wrote in message
...

** Hi to all the amp techs.

Bugera is an alias for Behringer - used for marketing their valve amps.

A model " 1990 Infinium " is on my bench now - first time I've seen one.

You need to see this Vid to get the background:

http://www.youtube.com/watch?v=P8GxgtLSadw

That slow talking dude makes some pretty wild claims - like 20 times
normal valve life, successfully mixing up different output types and care
free user maintenance. All are purest marketing fantasy.

The reality is very, very different.

The model referred above has a DSP based bias servo system that attempts
to set the idle current in each output tube at 34mA. Each cathode circuit
has a 4.7ohm resistor to ground to aid this process - shunted by a 3A
diode to protect it.

The DSP trims the four grid voltages to get 155mV DC across each
resistor - but only when there is no signal. The grid voltages become
locked at the previous idle values when even the smallest AC voltage
component appears at the cathodes.

There is enough bias voltage range available to cope with any EL34, 6L6 or
5881 ever made, from -65V to about -25V.

But this is complete madness, simply matching the idle bias current does
not make a class AB output stage work perfectly - not when the tube's
characteristics are quite different.

But there is worse.

The DSP bias servo adjusts for variations in AC supply voltage too. If the
AC supply voltage drops low when the amp is silent and then rises to
normal during playing, all the output tubes becomes seriously over biased.
Using a Variac, it is easy to create a situation were the amp self
destructs in seconds.

But there is worse.

The negative 75V DC bias supply is generated by a voltage tripler working
off the same transformer winding as the pre-amp tube heaters. All six
heaters are wired in series across a +/- 18V DC supply fed from a 20-0-20
AC winding on the mains tranny.

A pair of T1A fuses protect the above winding - but are fatally
undersized, fail easily at switch on and disable the bias supply
completely.

That is how I received the near new amp I have now - the AC fuse kept
blowing since the four Chinese made 5881s ( as originally fitted ) drew
max possible current as soon as they warmed up.

BTW 1:

There is a F6.3A fuse in series with the 6.3V AC heater supply for the
four output tubes - which is simply nuts. Even the T1.6A AC supply
fuse is under sized, since the amp draws 2.1 amps RMS at full drive.

BTW 2:

There is a plethora of utter nonsense posted about these models on net
forums, alleging bad tubes were fitted and / or fuses were defective.

The folk who know the truth are mostly contracted to Behringer / Bugera
and cannot say a thing.





.... Phil



Interesting. He starts off talking about tone, tone, tone, and that is true.
But the 'wanted' tone to one guitarist can be a mile different from that
wanted by another, and that is often achieved by altering the way that the
output stage is biased. Most of the guitar players that I know don't give a
**** about valve life, as long as they are getting the sound they want.
There are plenty of respected repairers and amp gurus giving on-line
tutorials about biasing for sound. So what is this 'ideal' bias point that
he is talking about ? Presumably a 'technical' ideal, because for guitar
players - particularly muso ones - the 'ideal' point is the one that makes
the output stage sound like they want it to sound. I know one muso that has
his amps' bias set very 'soft' to get the fat blues-y sound that is right
for what he plays. I know another that always has his set very 'hard' to get
a harsh screaming tone for the metal rock that he plays.

As to subbing the valves with anything that comes to hand, I can't help but
think that is a bad idea. If I had one valve out of four fail just before a
gig, I think I'd rather see the amp with that valve's opposite number
removed as well, and the volume turned up a bit - if that was even required.

Arfa